The resulfurized free cutting steel contains a large amount of oxygen to control the form of sulfide effective in machinability, i.e., to make the form of sulfide like a spindle. However, since all the oxygen cannot be dissolved in the sulfide, it is unavoidable for gigantic oxide to be formed so as to cause streak flaws, thereby generating surface flaws in the hot rolling step.
As techniques for solving the phenomena described above, there are proposed techniques that decrease the amount of oxide by lowering the oxygen content or lowering the content of Si serving as a deoxidizing agent (Patent Documents 1, 2, and 3). Further, there is proposed a technique that increases the dissolved oxygen by, increasing the amount of sulfide (Patent Document 4).
Patent Document 1 discloses a free cutting steel that contains a decreased quantity of gigantic oxide inclusions, while the oxygen content is set to be 0.008% or less. This document discloses that, in order to prevent the machinability from being deteriorated due to the lower oxygen content, an element for improving the form of sulfurized substances (sulfide) or an element for improving the machinability is added, or the rolling temperature is controlled. Consequently, the form of sulfurized substances (sulfide) is further improved, so that internal defects and/or flaws are prevented from being generated due to the gigantic oxide inclusions.
Patent Document 2 discloses a Pb-added free cutting steel applicable to shafts for OA equipment. This document discloses a component composition where the content of Si, which lowers the cleanliness of steel ingots, is set to be 0.1% or less, so as to decrease the amount of oxide. Further, in this composition, Cr content is set at 11.0% to mainly ensure the corrosion resistance, while the content of S, which deteriorates the corrosion resistance and hot workability, is set to be 0.01% or less.
Patent Document 3 discloses a low carbon resulfurized free cutting steel having good machinability. This document discloses a chemical component where the Si content is set to be 0.1 mass % or less, because SiO2, which is hard oxide harmful to the machinability, is remarkably increased if the Si content exceeds 0.1 mass %.
Patent Document 4 discloses an inexpensive free cutting steel to which Pb is not added. This document discloses a chemical component where a large amount of S is added to increase the total volume of sulfide, so as to greatly improve the free-cutting capability in the Pb-non-added type with lower Si and higher P. Further, the Mn/S is set to be larger than a certain value to prevent the hot workability from being deteriorated.
The free cutting steel disclosed in Patent Document 1 sets the oxygen content to be 0.008 mass % or less, but this merely decreases the oxygen content, and cannot sufficiently control the form of sulfide, thereby allowing the sulfide to be elongated. The free cutting steels disclosed in Patent Documents 2 and 3 set the Si content to be 0.1 mass % or less, but this merely utilizes S as a deoxidizing agent, and thus is not directed to a component composition with a particularly attention to improve the machinability. Further, the free cutting steel disclosed in Patent Document 4 contains a large amount of S, but the form of sulfide is not controlled.
Accordingly, the free cutting steels disclosed in Patent Documents 1 to 4 are still insufficient in machinability.
[Patent Document 1]    Jpn. Pat. Appln. KOKAI Publication No. 1-309946
[Patent Document 2]    Jpn. Pat. Appln. KOKAI Publication No. 9-176799
[Patent Document 3]    Jpn. Pat. Appln. KOKAI Publication No. 7-173574
[Patent Document 4]    Jpn. Pat. Appln. KOKAI Publication No. 2000-160284